Many hydraulic systems have a large capacity pump as the primary source of fluid for several control valves, each of which controls fluid flow to one or more fluid motors. In some systems, a smaller capacity pump is also provided to supplement the output of the larger pump when the fluid pressure of the system is below a predetermined magnitude. One of the problems encountered with such systems is that although the output of the small pump generally is needed for only the motor or motors controlled by one control valve or for only a small percentage of the time the output of both pumps passes through the control valves all the time that the fluid pressure is below the predetermined magnitude. This causes additional problems particularly when fluid flow to the motors is being modulated by the control valves since the control valves must then modulate the total output of both pumps.
The Applicant of the present application is aware of one system which solves the above problem. Such system has a fluid circuit which connects a first pump to a fluid motor and has a control valve for controlling fluid flow from the first pump to the motor. The control valve has an inlet port connected to the pump, a motor port connected to the motor, and a valve spool movable between a first position at which the inlet port is blocked from communication with the motor port, a second position at which fluid flow from the inlet port to the motor port is controllably modulated and a third position at which substantially the total output of the first pump passes from the inlet port to the motor port. The second pump is connected to the fluid circuit through a combining valve in response to the valve spool being moved to the third position. At the third position of the valve spool, fluid pressure is directed from the signal port to a signal chamber of the combining valve to move a valve member to an actuated position while a spring returns the valve member to the nonactuated or noncombining position when the signal chamber is vented. However, it has been found in practice that it is desirable to at least have the capability under some operational conditions of moving the valve member in both directions by fluid pressure in case the valve member should stick.